Executive Summary
Cascades
Diversion Dam is a timber crib dam constructed in 1917 to divert the flow of
the main stem of the Merced Wild and Scenic River into a hydroelectric
generating facility (see photo). The hydroelectric facility was taken off line
in 1985 and 1986 because it was deteriorated and outdated. Turbines and other
equipment were removed from the powerhouse, and the majority of the penstock
was removed. Some elements of the former hydroelectric generating facility
still remain, including abutments that flank the dam, an intake structure, a
screenhouse, 300 feet of concrete penstock (which historically conveyed water
from the dam to the powerhouse), the powerhouse, and the transmission lines.
Cascades Diversion Dam impedes flow of the Merced Wild and Scenic River
in this area. In addition, Cascades Diversion Dam is classified as a high-hazard
structure (USBR 1997). It is in unsatisfactory condition due to flood damage sustained
in 1997 and to continuing deterioration associated with age. Due to the threat
to public health and safety and to natural resources in the Merced Wild and
Scenic River corridor, the dam needs to be removed before further deterioration
or total dam failure occurs.
Cascades Diversion Dam and intake structure with screens
were determined to be eligible for nomination to the National Register of
Historic Places in 1982 (NPS 1982). The National Park Service has completed all
cultural resource compliance procedures required under the National Historic
Preservation Act for removal of the dam and associated features (NPS 1987b).
Refer to Chapter VI, Consultation and Coordination, for a history of
environmental compliance related to Cascades Diversion Dam.
The National Park Service solicited public input on the
proposed project by holding a public scoping meeting, accepting public scoping
comments, publishing fact sheets and planning updates, and through ongoing open
house meetings. Based on federal law, regulations, and executive orders, and
public scoping comments, the National Park Service determined that an
environmental assessment (not an environmental impact statement) would be the
appropriate level of compliance for the Cascades Diversion Dam Removal Project.
Public scoping comments and issues raised by National Park Service staff were used
in the alternatives development process and the analysis presented in this
document. As lead agency for the National Environmental Policy Act process, the
National Park Service is responsible for preparation of this environmental
assessment.
The purpose of the Cascades Diversion Dam Removal Project
is to remove an unnatural obstruction on the Merced River
and to restore the river’s natural free-flowing condition. This removal project
is consistent with the Wild and Scenic River guidance provided in the Merced
Wild and Scenic River Comprehensive Management Plan (Merced River Plan) (2001a) and will meet the direction of the Yosemite
Valley Plan (NPS 2000a), which calls for the dam’s removal.
Cascades Diversion Dam is classified as a high-hazard
structure (USBR 1997). It is in unsatisfactory condition due to flood damage
sustained in 1997 and to continuing deterioration associated with age. In
addition, the dam no longer serves a useful purpose – water is not
diverted from the site to generate electricity or for other uses, and the
impoundment does not regulate high water. Removal of the existing dam structure
is necessary to prevent possible uncontrolled and sudden failure, which could
result in a release of impounded water and the deposition of concrete and
timber debris, grouted rockfill, and impounded sediment along the downstream
channel. Such an occurrence could pose a considerable threat to valued
resources (such as aquatic life, scenic vistas, and recreational
opportunities), infrastructure (El Portal Road,
wastewater, telephone, and electrical lines), and human life.
In addition, the National Park Service is entrusted with
conserving and restoring park values. This responsibility includes protecting
the biological and physical processes that created the park, along with scenic
features, natural landscapes, and native plants and wildlife. The removal of
the dam would work toward fulfilling this mandate by restoring this segment of
the Merced River.
The Yosemite National
Park 1980 General
Management Plan, Merced River Plan, and Yosemite
Valley Plan are the guiding documents for the Cascades Diversion Dam
Removal Project, which is located within the Wild and Scenic
River boundaries of the Merced
River. The General
Management Plan is the overall guiding document for planning in Yosemite
National Park. The Merced River
Plan is a programmatic plan that derives its authority from the Wild and Scenic
Rivers Act. In designating the Merced River as a Wild and Scenic River,
Congress authorized the National Park Service to prepare a management plan for
the river by making appropriate revision to the park’s General Management Plan (16 United States Code 1274[a][62]). The
Cascades Diversion Dam Removal Project complies with conditions outlined in the
Merced River Plan. The Yosemite Valley
Plan is an implementation plan that presents a comprehensive management
plan for Yosemite Valley. The Cascades Diversion Dam
Removal Project would implement an action called for in the Yosemite Valley Plan. Specific actions
proposed by this project are consistent with guidance set forth by the General Management Plan, the Merced
River Plan, and the Yosemite Valley Plan.
The Cascades Diversion Dam Removal Project Environmental
Assessmentpresents and analyzes three
sets of proposals, referred to as the alternatives. The No Action Alternative
represents management direction and conditions as they currently exist for
Cascades Diversion Dam. The two action alternatives represent a reasonable
range of options to satisfy the purpose of and need for the project, while also
meeting all relevant legal requirements. Each of the action alternatives aims
to achieve the goals of this project, but varies in its proposal for Cascades
Diversion Dam.
The National Park Service has identified Alternative 2, Complete
Dam Removal, as the preferred alternative. It protects resources, meets the
direction of the Yosemite Valley Plan,
complies with the goals of the General
Management Plan and Merced River Plan, and reflects the spirit of the Wild
and Scenic Rivers Act. Meeting these needs complies with the National Park
Service Organic Act and Yosemite National
Park enabling legislation. The selection of a
final alternative will be documented in a Finding of No Significant Impact.
Alternative 1: No Action
The No Action Alternative maintains the status quo at
Cascades Diversion Dam, as described in Chapter III, Affected Environment. It
provides a baseline from which to compare the action alternatives, to evaluate
the magnitude of proposed changes, and to measure the environmental effects of
those changes.
Under the No Action Alternative, Cascades Diversion Dam
would continue to degrade and would eventually fail. Dam failure would likely
occur during high-flow conditions, releasing large debris and sediments to the
river and banks. In addition, continued deterioration of the dam over time
would result in the release of large debris. Dam debris could damage downstream
natural, cultural, and scenic resources as well as recreation opportunities and
park facilities. Dam debris could also result in serious injury and/or fatality
to recreational users of the river. There are voids under the existing timber
crib sheathing, and many boards are on the verge of collapse. People walking on
the dam’s wooden crest and the exposed riverbanks and rocks in close proximity
to the dam could be exposed to hazards, such as falling from the dam structure.
The National Park Service would remove dam debris from the river as soon as
possible following release, although debris retrieval would not likely commence
until low-flow conditions, which could be several months after dam failure or
debris release. Diverted riverflows and erosion could result in adverse impacts
to vegetation, soils, and cultural resources along the riverbanks. Over the
long term, uncontrolled dam failure would restore free flow of the Merced
River at this location.
Alternative 2: Complete Dam Removal
Alternative 2 includes complete removal of the dam, the
dam abutments, the intake structure, and the screenhouse, and restoration of
the related river channel located beneath the dam site. Approximately 4,400 to
5,400 cubic yards of sediments (including rock and boulders) in the area
upstream of the dam would be excavated and repositioned to stabilize the
river-right bank and decrease the potential for sediment erosion. Natural river
processes would continue to transport remaining sediments (up to a maximum
range of approximately 9,600 to 15,600 cubic yards of sediment) from the
impoundment area over time, allowing for a gradual re‑establishment of
the natural river channel and related riparian habitat. It is expected that the
river would fully recover over time, as sediments are transported from the
impoundment area. However, the rate of natural channel recovery and restoration
would be monitored to determine if additional restoration actions were
necessary. Following removal of the river-right abutment, intake structure, and
screenhouse, the river-right bank would be stabilized using a bioengineered bank stabilization system
(brush layering incorporated into a boulder structure) to prevent erosion. The
objective of this alternative would be to restore the natural river character
with a mixture and distribution of boulders, cobbles, gravels, sand, silt, soil,
and vegetation similar to those found in adjacent riverbank segments.
Alternative 3: Partial Dam Removal
Alternative 3 includes complete removal of the dam, the river-left
dam abutment, and the screenhouse on the river-right intake structure, and
restoration of the related river channel located beneath the dam site. Under
this alternative the river-right dam abutment and intake structure would be
retained for use as a river viewing platform. Approximately 4,400 to
5,400 cubic yards of sediments (including rocks and boulders) in the area
upstream of the dam would be excavated and repositioned to stabilize the
river-right bank and decrease the potential for sediment erosion. Natural river
processes would continue to transport remaining sediments (up to a maximum
range of approximately 9,600 to 15,600 cubic yards of sediment) from the
impoundment area over time, allowing for a gradual re-establishment of the
natural river channel and related riparian habitat. It is expected that the
river would fully recover incrementally over time, as sediments are transported
from the impoundment area. However, the rate of natural channel recovery and
restoration would be monitored to determine if additional restoration actions were
necessary. Following removal of the dam and screenhouse, the river-right bank
would be stabilized upstream and downstream of the intake structure using a
bioengineered bank stabilization system to prevent erosion of the river-right
bank. The objective of this alternative would be to restore the natural river
character with a mixture and distribution of boulders, cobbles, gravels, sand,
silt, soil, and vegetation similar to those found in adjacent riverbank segments.
The Council on Environmental Quality Regulations
implementing the National Environmental Policy Act and the National Park
Service National Environmental Policy Act guidelines require that “the
alternative or alternatives which were considered to be environmentally
preferable” be identified (Council on Environmental Quality Regulations,
Section 1505.2). Environmentally preferable is defined as “the alternative that
will promote the national environmental policy as expressed in the National
Environmental Policy Act’s Section 101. Ordinarily, this means the alternative
that causes the least damage to the biological and physical environment; it
also means the alternative that best protects, preserves, and enhances historic,
cultural, and natural resources” (Council on Environmental Quality 1981).
Section 101 of the National Environmental Policy Act
states that “… it is the continuing responsibility of the Federal Government
to … (1) fulfill the responsibilities of each generation as trustee
of the environment for succeeding generations; (2) assure for all
Americans safe, healthful, productive, and aesthetically and culturally
pleasing surroundings; (3) attain the widest range of beneficial uses of
the environment without degradation, risk to health or safety, or other
undesirable and unintended consequences; (4) preserve important historic,
cultural, and natural aspects of our national heritage, and maintain, wherever
possible, an environment which supports diversity, and variety of individual
choice; (5) achieve a balance between population and resource use which
will permit high standards of living and a wide sharing of life’s amenities;
and (6) enhance the quality of renewable resources and approach the maximum
attainable recycling of depletable resources.” The environmentally preferable
alternative for the Cascades Diversion Dam Removal Project is based on these
national environmental policy goals.
Alternative 1: No Action
The No Action Alternative represents conditions and
management practices as they currently exist for Cascades Diversion Dam. The
provision of safe, healthful, productive, and aesthetically and culturally
pleasing surroundings (provision 2 of the national environmental policy goals)
would be adversely affected due to continued deterioration and uncontrolled dam
failure, which would result in visually intrusive bank erosion and debris
within the channel of the Merced River, endanger the downstream aesthetic and
cultural resources in the vicinity, and result in short-term air quality,
noise, transportation, and orientation and interpretation impacts. However,
Alternative 1 would partially fulfill provision 2 by continuing to provide a carpool/recreation
access point in the project vicinity. Alternative 1 would not fulfill
provision 3 of the national environmental policy goals, because risks to public
health and safety would worsen under this alternative (due to the uncontrolled
failure of the dam and continued safety hazards associated with falling from the
dam structure) and because recreation resources could be impacted by dam
failure and debris retrieval activities. Alternative 1 would not preserve
natural resources as required under provision 4 of the national environmental
policy goals. Eventual dam failure would lead to sudden bank erosion that would
affect soils, water quality, biological resources such as vegetation and
special-status species, and cultural resources downstream from the dam. Under
Alternative 1, the dam (a contributing element to the Yosemite Hydroelectric
Power Plant) would no longer exist.
Alternative 2: Complete Dam Removal
Alternative 2 includes the complete removal of the
diversion dam and attendant structures in addition to site restoration. Because
dam removal would occur in a controlled manner (e.g., within a delineated work
area, during low-flow conditions, with the application of best management
practices), Alternative 2 would avoid the more pronounced adverse effects of
uncontrolled dam failure and debris retrieval activities described under
Alternative 1. The application of mitigation measures described in Chapter II
would further reduce the potential adverse impacts. Site restoration and bank
stabilization under this alternative would aesthetically improve the project
area surroundings (provision 2 of the national environmental policy goals). Alternative
2 would fulfill provision 3 of the national environmental policy goals by
reducing risks to public health and safety through the controlled removal of
the dam and application of mitigation measures to reduce hazards to visitors. Alternative
2 would also retain the parking lot to the north of the dam, but would remove
health and safety impacts to recreation users (conflict with vehicles)
associated with use of the dam area for sightseeing and other activities. Alternative
2 would preserve natural and downstream cultural resources as required under
provision 4 of the national environmental policy goals. This alternative would
implement measures to reduce adverse effects related to dam removal activities
(e.g., best management practices) and includes site restoration and bank
stabilization to increase site stability and biological integrity. Alternative 2
would also ensure protection of sensitive cultural resources in the project vicinity
by avoiding the adverse effects of downstream bank erosion that would occur
under Alternative 1. As with Alternative 1, the dam (a contributing element to
the Yosemite Hydroelectric Power Plant) would no longer exist under Alternative
2.
Alternative 3: Partial Dam Removal
From an environmental perspective, Alternative 3 differs
from Alternative 2 primarily in its retention of the river-right dam abutment
and intake structure as a river-viewing platform. Alternative 3 would be less
sensitive than Alternative 2 to provisions 2, 3, and 4 of Section 101 of
the National Environmental Policy Act, as stated above, because free flow would
remain affected in the immediate area of the structure; therefore, Alternative
3 would not enhance the integrity of wetland and aquatic resources in the area
to the same degree as Alternative 2. The health and safety of park visitors
would not be protected to the same degree as under Alternative 2, due to
continued vehicle safety hazards. Further, the river-viewing platform (and
visitors using the platform) would be exposed to long-term geologic hazards. Removal
of the parking area to the north of the dam would eliminate a carpool/recreation
access point in the project vicinity. As with Alternative 1, the dam (a
contributing element to the Yosemite Hydroelectric Power Plant) would no longer
exist under Alternative 3.
Environmentally Preferable Alternative
The environmentally preferable alternative is Alternative
2 because, among the alternatives considered in detail, it most fully satisfies
the national environmental policy goals stated in Section 101 of the
National Environmental Policy Act. Alternative 2 would (1) provide a
high level of protection of natural and downstream cultural resources while
concurrently attaining the widest range of beneficial uses of the environment
without degradation; (2) reduce risks to public health and safety; and (3) provide
aesthetically pleasing surroundings.
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